Private reply
Also called: Instagram private replies API
A private reply is the single DM Meta lets a Professional Instagram account send to someone who commented on its post, Reel or Live, within 7 days of the comment.
One comment, one message
Meta's documentation allows one private reply per comment, sent within 7 days of the comment being made. That message is the whole allowance. If the person answers it, a normal conversation opens and the account can reply within 24 hours of their message.
This is why an automated follow-up sometimes never arrives: if the commenter read the first DM but never replied, there is no second message to send. A tool that appears to queue one is either dropping it or breaking the rule.
Live is different
For Instagram Live, private replies can only be sent while the broadcast is running. Once the Live ends, comments on it can no longer receive a private reply.
Message limits
Text sent through the Instagram Messaging API must be UTF-8 and at most 1,000 bytes. Emoji and scripts such as Devanagari, Arabic or Thai use two to four bytes per character, so a message in those languages fits fewer characters than one in English.